The present invention relates to a device intended to ensure protection of electrical apparatus against voltage surges which sometimes occur on the conductors used in electric energy distribution networks, particularly in the event of lightning striking.
The prior state of the art has proposed a very diverse quantity of lightning arrester devices which are connected between a wire of the network and earth or a common conductor element, and which comprise components which, in normal functioning, act like neutral elements but which, in the event of violent voltage surge, become conducting so that they divert the current due to the voltage surge and in particular to the lightning, thus protecting the installation disposed downstream against the destructive effects of such voltage surge.
The technical characteristics which are required of such apparatus are very varied, with the result that, up to the present time, no lightning arrester device on the market totally satisfies all the necessary characteristics:
The latter are the following:
very short response time,
very great power of flow of the current,
very low residual voltage,
continuity of the service, and of the protection,
cost price having to be able to be determined as a function of the cost of the apparatus and of the devices to be protected.
In order to perform such a function, it has been proposed to use gas discharge arresters. However, these latter possess noteworthy drawbacks and in particular that of presenting a considerable delay at ignition so that, in certain cases, and especially in the case of particularly violent and rapid voltage surges provoked by lightning, the electronic apparatus deteriorate before the lightning current is diverted by the discharge arrester.
Varistors have also been employed which present the advantage of absorbing considerable thermic and electric energies without being destroyed, when they are subjected to current impulsions, so that they thus make it possible to peak-clip a voltage surge, particularly provoked by lightning, without the supply of current of the apparatus that the device intends to protect being interrupted. An example of a characteristic of varistor is represented in FIG. 1 (curve b).
However, it is generally known that a lightning arrester is a device which is connected between a line to be protected and earth or a common conductor element, and which becomes conducting when a transitory voltage surge occurs and which, by flowing off the energy therefrom, protects the sensitive electronic equipment disposed downstream, against the destructive effects of this voltage surge. It is thus understood that a lightning arrester of ideal type must not conduct the electric current when the voltage at its terminals is normal and must rapidly conduct a large quantity of current when the voltage becomes abnormal, while maintaining the voltage surge at an acceptable level.
Now, the principal drawback of varistors is that, under the effect of repeated electric shocks, and by reason of the high operational temperatures, they are subjected to wear which is translated by a resistive fast-blow by lowering of the peak-clipping threshold. One is thus obliged to introduce disconnectors (standard NFC 61-740 July 1995 and standard CEI 61653-1 of Mar. 1, 1998) in the energy lines, whose purpose is to open the circuit in the case of fast-blow in order to avoid a fire being started. One of the disconnectors used is obligatorily disposed inside the body of the lightning arrester, which brings about elimination of the protection as soon as it is actuated.
Such a mode of operation is extremely risky at the level of protection of an installation insofar as, during a storm, the voltage surges follow one another at a certain rhythm depending on the different thunder claps. Various means intended to overcome this drawback have therefore been proposed, particularly by disposing a plurality of varistors in parallel, which is possible thanks to the importance of the slope (U/I ratio) of their characteristic. Such a placing in parallel may be effected with the aid of varistors of different values, which makes it possible to monitor, to a certain extent, the order of their destruction.
It is also known that another drawback of the varistors is that they present too high a residual voltage in the case of a very rapid voltage front and in the case of very strong currents.
Lightning arrester devices have also been proposed, employing Zener diodes which present the advantage of not being influenced by wear due to repeated electric shocks and of operating at high temperatures. These lightning arresters are such that, once the Zener diode is destroyed due to a voltage surge, they immediately become conducting, with the result that they conduct the voltage surge to earth. In that case they present a considerable capacity of flow of the currents and very weak residual voltages, and this in all cases.
Unfortunately, lightning arresters of this type present the drawback of being of a high cost, insofar as the Zener diodes used are constructed from monocrystalline silicon which is much more expensive than the products used for producing the varistors and which must be disposed between heat-dissipating discs made of copper or silver. Such lightning arresters must therefore be constructed with a large number of discs which is a function of the magnitude of the energy that it is desired to absorb before destruction of the element in short-circuit.
Nonetheless, despite their cost, such lightning arresters are, however, interesting, since, after they are destroyed, they ensure a continuity of the protection of the apparatus that they are intended to protect.
Patent DE-U-298 08 365 also discloses a high voltage protection circuit intended to protect low input impedance measuring devices making it possible to avoid different problems which are encountered on the conventional circuits and in particular current loss effects during high-precision measurements.
This circuit, whose purpose is different from the one forming the subject matter of the present application, comprises a varistor and a Zener diode which are associated. However, the terminals of these two elements are not common and the circuit can therefore not perform the function of the circuit according to the invention.